Ray tracing - color mixing

Question

I am writing a ray tracer. So far, I have diffuse and specular lighting, and I am planning to implement reflection and refraction, too.

So far I have used white lights, where I calculated the surface color like this: surface_color * light_intensity, divided by the proper distance^2 values, since I am using point light sources. For specular reflection, it's light_color * light_intensity. Afaik, specular reflection doesn't change the light's color, so this should work with different color light sources, too.

How would I calculate the color reflected from a diffuse surface when the light source is not white? For example, (0.7, 0.2, 0) light hits (0.5, 0.5, 0.5) surface. Also, does distance factor in differently in this case?

Also, how would I add light contributions at a single point from different color light sources? For example, (1, 0.5, 1) surface is lit by (0.5, 0.5, 1) and (1, 0.7, 0.2) lights. Do I simply calculate both (distances included) and add them together?

Answer 1

• I've found that RGB is a poor color space to do lighting calculations in because you have to consider a bunch of special cases to get anything that looks realistic or behaves the way you would expect it to.
• With that said, it may be conceptually easier to do your lighting calculations in HSL rather than RGB. Depending on the language and toolkit you're using, this should be part of the standard library/distribution or an available toolkit.
• A more physically accurate alternative would be to implement spectral rendering, where instead of your tracing functions returning RGB values, they return a sampled spectral power distribution. SPDs are more accurate and easier to work with than keeping track of a whole bunch of RGB blending special cases, at the cost of a slight but noticeable performance hit (especially if left unoptimized). Specular highlights and colored lights are a natural consequence of this model and don't require any special handling in the general case.